Light-emitting diodes (LED) are semiconductor light sources traditionally used as indicator lamps in many devices. In addition, LEDs are being increasingly used also for lighting, where one particular use is for providing backlighting. For example, LED backlighting is being increasingly used for liquid crystal displays (LCDs), as LCDs do not produce their own illumination. Furthermore, LED backlight lighting systems are becoming increasingly common for the use in display backlighting and keypad backlighting in portable devices such as cell phones, smartphones, PDAs, digital cameras, personal navigation devices and other portable devices with keypads and/or LCD displays.
LED lighting systems are generally associated with a variety of advantages over traditional lighting sources such as incandescent lighting. For example, LEDs are efficient, associated with longer life, exhibit faster switching and produce less heat than traditional lighting sources. Due to the faster switching characteristics of LEDs, they are suitable for use in fast and highly responsive circuits by allowing for both quick response/start-up time and the capability to be operated at high frequency, further allowing for such enhancements as frequency modulation in order to reduce power consumption.
LED lighting systems typically comprise “strings” of stacked LEDs (also referred to as LED strings or LED circuits in the following) in which multiple LEDs are connected in series. An LED driver control circuit provides a regulated high supply voltage to the LED strings of stacked LEDs. A common practice to control the current that flows in each of the LED strings is to pull a well-defined current from the cathode side of each LED string, via programmable current sources or programmable current sinks. In order to protect the system components from excessive voltage levels and avoid excessive high current to flow in the LED circuit, an overvoltage protection mechanism is generally provided to disable the delivery of power to the circuit in the event that the voltage rises above a certain threshold.
FIG. 1 schematically illustrates a conventional LED lighting system 100. This LED lighting system 100 comprises a plurality of LED strings 101 (of which only one is shown in FIG. 1 for illustrative purposes), each LED string 101 comprising a plurality of LEDs 102. Typically, a LED string 101 may comprise up to six or more LEDs 102, and the LED lighting system 100 may comprise up to 12 or more LED strings 101. An IDAC (current digital-to-analog converter) current generator 110, which is an example of a programmable current sink, is provided for each LED string 101 at the cathode side of the respective LED string 101. The IDAC current generators 110 allow sinking/pulling a well-defined current from each LED string 101 to ground. The LED lighting system 100 illustrated in FIG. 1 further comprises a boost regulator circuit 120 (power source) that comprises a boost controller 130 and is adapted to provide a regulated boost voltage (supply voltage, drive voltage) to each of the LED strings 101, or in more detail, to a boost voltage node 105 at the anode side of each LED string 101. A feedback voltage from a feedback node 106 at the cathode side of each LED string 101 is provided to the boost regulator circuit 120. The boost regulator circuit 120 is adapted to boost a battery voltage to a supply voltage that is higher than the battery voltage and to regulate the voltage at the feedback node 106 at the cathode side of each LED string by performing feedback control in accordance with the feedback voltages received from the LED strings 101.
In the LED lighting system 100 illustrated in FIG. 1, each LED string's 101 current ILED is programmed by a respective IDAC current generator 110. The voltage drop (overhead) across each IDAC current generator 110 (i.e. the voltage drop between the feedback node 106 and ground) times the respective current, multiplied by the number of LED strings 101, results in a dissipation of power which should be minimized in order to increase the overall boost efficiency.
In more detail, the total dissipated power resulting from the overhead consumption is given byPdiss=VLED·ILED·Nstr,where VLED is the voltage at the feedback node 106, ILED is the current flowing through each LED circuit 101, and Nstr is the number of LED circuits 101. Here, and in the following, unless not indicated otherwise, all voltages are understood to be given with reference to ground. Thus, in order to increase the efficiency of the boost regulator circuit 120, the overhead needs to be reduced.
Prior art lighting systems employ a plurality of current mirrors, one or more for each LED string 101, for sourcing the well-defined current to each LED string 101. Each current mirror is configured to mirror a respective reference current to the cathode side of the respective LED string 101. Conventionally, cascoded current generators that work in saturation are used for forming the current mirrors.
These lighting systems have the drawback that a high voltage drop over the current generators occurs, i.e. that a high overhead is present, at the cost of system efficiency. In addition, conventional lighting systems have poor scalability as regards quiescent current consumption and also as regards area of circuitry. A further drawback of conventional lighting systems is that properties (such as e.g. threshold voltages and/or resistances) of the transistors used in the current mirrors may deviate from expected properties due to imperfect manufacturing processes, during which e.g. doping mismatches occur. As a result, the values of the currents flowing through each LED circuit may deviate from expected values, further degrading the overall performance of the lighting system.